1. Field of the Disclosure
The present application relates to an ophthalmological measurement apparatus and an ophthalmological measurement method for determining the axial length of an eye. The present application relates, in particular, to an ophthalmological measurement apparatus and an ophthalmological measurement method for contactlessly determining the axial length of the eye.
2. Related Art
As is explained, for example, in U.S. Pat. No. 5,347,328, conventional apparatuses for the interferometric measurement of the axial eye length, in which laser beams having a parallel beam profile converging on the retina are radiated onto the eye, have the disadvantage that the light reflected by the retina can be received outside the eye with a parallel beam path, whereas the light reflected by the cornea is reflected divergently through the cornea such that different wavefront profiles result for the light reflected by the retina and the light reflected by the cornea. During the detection of interference patterns with the aid of light receivers of small area, the different wavefront profiles result in only weak signals which can scarcely be distinguished from the noise caused by the rough retina surface, and during the detection with the aid of light receivers of large area a number of interference patterns result which reduce the signal modulation and therefore also do not permit unique conclusions relating to the distance between cornea and retina. In order to overcome these problems, in accordance with U.S. Pat. No. 5,347,328 the light reflected by the cornea and the light reflected by the retina is fed to different optical light acquisition systems by means of a beam splitter. The light received in the different light acquisition systems is combined, and interference is determined, in an optical interference light receiver. In accordance with U.S. Pat. No. 5,347,328, it follows that two differently dimensioned optical illumination and acquisition systems are required for acquiring the light reflected at the cornea and the light reflected at the retina, and for the interferometric determination, based thereon, of the axial eye length. Furthermore, avoiding parasitic inference requires the use of a path length compensation which sets the acquisition systems to exactly the same optical path lengths. It can be said in general that the simultaneous acquisition of retina and cornea requires twice as high an outlay on hardware by comparison with the simple Michelson interferometer. Avoiding measuring errors owing to axial eye movements during the measurement procedure requires simultaneous acquisition. U.S. Pat. No. 5,347,328, as well as U.S. Pat. No. 6,779,891 make use for this purpose of interferometric designs which can determine only the relative spacing of optical interfaces, but not the distance from the measuring instrument. When there are more than two optical interfaces, their position relative to one another cannot be determined uniquely. Were such measurement methods to be used to determine further intraocular distances, it would, for example, be impossible to distinguish between the anterior chamber depth and the lens thickness.